This invention relates generally to conveyor systems, and more particularly to controls for a gravity conveyor.
A wide variety of conveyors are used to transport packages and other types of articles throughout warehouses and factories. Many of these conveyors are powered by motors that turn belts, rollers, slats, or other supports on which the products rest, thereby causing the products to move along the length of the conveyor. In one type of conveyor, known as a gravity conveyor, the conveyor is not powered, but rather is slanted downwardly and uses the force of gravity to cause products to move down the length of the conveyor. Usually the gravity conveyor includes a plurality of rollers which allows the products to roll down the conveyor on top of the rollers.
Gravity conveyors are often used at the end of a conveyor system. After the products roll down the gravity conveyor, the conveyor usually terminates with a bar, a case stop, or other means that are designed to prevent the packages from rolling off of the end of the conveyor. If the products are not lifted off of the conveyor at this point, they begin to accumulate at the bottom of the gravity conveyor and proceed to accumulate up the gravity conveyor as more packages arrive. In prior gravity conveyors, the packages that accumulate on the gravity conveyor exert a force on downstream packages due to gravity. When enough packages accumulate, this force can squeeze the package at the very end of the conveyor tightly against the bar, case stop, or other means that stop the packages. This squeezing force can significantly increase the difficulty of lifting the end package off of the conveyor because, in addition to the weight of the package, a person lifting a tightly squeezed package has to overcome the additional frictional forces from the adjacent upstream package and the stop. As more packages accumulate, the squeezing force increases, making the lifting process even more difficult and, in some cases, crushing the packages at the end. This is, of course, undesirable.
Prior gravity conveyors that are not controlled also suffer from the disadvantage that the speed of packages rolling down the gravity conveyor is not limited. Packages may therefore accelerate sufficiently to either themselves fall off the end of the conveyor, knock other packages off of the end of the conveyor, or both. Fast traveling packages coming down a gravity conveyor can also cause problems of pinching or injuring the hands of workers who lift the packages off of the end of the conveyor. Fast traveling can also cause severe damage to the contents of the packages. If a package containing fragile material travels down the conveyor right before a heavier package following it, the uncontrolled heavier package can easily damage the contents of the package with the fragile material. This increases costs and oftentimes customer dissatisfaction.
In the past, some of these disadvantages have been overcome by controlling the gravity conveyor through the use of brakes that selectively prevent one or more rollers on the gravity conveyor from turning. When the rollers on the conveyor are braked, the packages on top of the braked rollers tend to stop (absent slippage over the roller). In one prior art pneumatic system, a portion of the gravity conveyor includes brakes that are oscillated on and off at manually adjustable periods. By alternating the brake or brakes on and off, this system causes the packages to move down the conveyor incrementally, thereby avoiding the excessive speeds that are associated with uncontrolled gravity conveyors. This system, however, oscillates the brake or brakes on and off regardless of the presence or absence of packages, which can cause the system to unnecessarily waste energy when no packages are present, or when the packages have accumulated and are not being removed from the end of the conveyor. Also, because this system only covers a small portion of the total length of the conveyor and never locks the brakes on, it will allow the fall weight of the accumulated packages to push against the end package, thereby failing to alleviate the problem of the end package being excessively squeezed. Furthermore, the adjustability of the pneumatic controllers is very difficult and time-consuming, especially if there are multiple gravity conveyors within a given warehouse or plant. The desirability of a controlled gravity conveyor that overcomes these and other disadvantages can therefore be seen.
Accordingly, the present invention provides an improved control system and method for controlling a gravity conveyor that is more energy efficient, that tends to reduce the squeezing forces that are applied to the end packages at the bottom of the gravity conveyor, and that tends to deliver articles to the bottom of the conveyor in a manner that is more responsive to the speed at which a worker or workers are removing packages from the end of the conveyor.
According to one aspect of the present invention, a method is provided for controlling a gravity conveyor having a plurality of rollers arranged in a descending manner from an upper end to a lower end such that articles move by force of gravity down the conveyor. The method includes providing a sensor adapted to detect the presence or absence of an article at the lower end of the conveyor. A brake is also provided and located upstream of the sensor. The brake is adapted to be turned on or off. In the on condition, the brake substantially prevents at least one associated roller from rotating. In the off condition, the brake allows the at least one associated roller to rotate. The brake is turned on when the sensor detects an article at the lower end of the conveyor.
According to another aspect of the present invention, a gravity conveyor is provided that includes a plurality of rollers arranged in a descending manner from an upper end to a lower end such that articles move by force of gravity down the conveyor. The conveyor further includes a sensor, a brake, and a controller. The sensor is adapted to detect the presence or absence of an article at the lower end of the conveyor. The brake is located upstream of the sensor and is adapted to be turned on or off. In the on condition, the brake substantially prevents at least one associated roller from rotating. In the off condition, the brake allows the at least one associated roller to rotate. The controller is adapted to turn the brake on when the sensor detects an article at the lower end of the conveyor.
According to yet another aspect of the present invention, a gravity conveyor is provided having a plurality of rollers arranged in a descending manner from an upper end to a lower end such that articles move by force of gravity down said conveyor. The conveyor includes a plurality of zones, at least one sensor associated with each zone, and a controller. Each of the plurality of zones includes at least one brake adapted to prevent at least one associated roller from rotating when turned on. Each of the sensors are adapted to detect the presence or absence of articles within the vicinity of the sensor. The controller is adapted to send a pulsed signal to each of the brakes to turn each of the brakes on. The controller is further adapted to stop sending the pulsed signal when each of the sensors has not detected a change in the absence or presence of an article for more than a predetermined amount of time.
In other aspects of the invention, the controller may be a programmable logic controller or other logic-following controls means and the sensor may be a photo-eye positioned along the side of the gravity conveyor. By using a programmable logic controller or other logic-following controller, the parameters that are used to control a single conveyor can be duplicated for other similar conveyors. This duplication can lead to significant savings in time and money during the implementation of gravity controlled conveyor systems. The brake or brakes may be air-actuated brakes that hold the roller or rollers when high pressure is applied to the brake and release the roller or rollers when the high pressure is terminated. The gravity conveyor may further be divided into two or more zones, each having a sensor and at least one brake. Based on the absence or presence of detected articles in each zone, the brakes in one or more of the zones may be selectively activated. The selective activation of the brakes may further be based on the initial detection of an article in a zone, the simultaneous detection of articles in two zones, the removal of articles from the downstream-most zone, a cyclical pulsing signal, or a combination of any one or more of these factors.
The methods and apparatuses of the present invention provide a method and control system for controlling a gravity conveyor that operates with improved energy efficiency, that substantially eliminates the problem of unduly squeezed packages at the bottom of the gravity conveyor, and that provides generally improved delivery of the packages to the worker or workers at the bottom of the conveyor. These and other advantages of the present invention will be apparent to one skilled in the art in light of the following specification when read in conjunction with the accompanying drawings.